1. Field of Invention
The present invention is generally concerned with milk quality analysis and in particular with an on-line, fully integrated somatic cell analyser.
2. Description of the Prior Art
The major cause of loss in dairy farming is an infection, known as mastitis, which occurs in an animal's udder. Mastitis is caused by contagious pathogens invading the udder and producing toxins that are harmful to the mammary glands. Generally, mastitis starts in one quarter.
Somatic cells, predominantly white cells and epithelial cells, enter the mammary gland as a result of damage to the alveolar lining by infection or chemical irritation. The counting of somatic cells excreted in the milk has become a widely used measure of mammary gland inflammation. The somatic cells can be counted by laborious direct microscopic method on stained milk smears, or the cell numbers can also be estimated by direct chemical tests. Other methods measure milk somatic cells indirectly or by determining the concentration of various by-products of the inflammatory response.
Somatic cell count (SCC), which is the number of white cells per milliliter of milk, increases in the bulk tank as mastitis spreads in the herd. SCC scores are used as an international standard in determining milk's quality and price. Most marketing organizations and regional authorities, regularly measure SCC on bulk tank milk and use theses scores for penalty deductions and/or incentive payments. High SCC scores indicate the presence of mastitis in the herd and is reflected in the average score of the bulk tank. The bulk tank SCC is a good indicator of overall udder health and a good means for evaluating the mastitis control program.
It is also a high correlation between the bulk milk SCC and the average of individual animal counts. It is not uncommon for a few problem animals to be responsible for greater than 50% of the somatic cells in the bulk tank, particularly in small herds. It should be noted that animals with high milk production and intermediate SCC levels can have a significantly higher percentage of SCC contribution to the tank score than some high SCC cows with low production.
For high quality milk the SCC should be less than 200,000 cells/ml. Acceptable milk has SCC scores from 200,000 to 500,000 cells/ml. For infected animals, milk SCC scores are between 600,000 to 1.2 M cells/ml.
When an animal in the herd becomes infected with infectious pathogens a rapid drop in milk production will be noted within 2 to 3 days. A high level of bacteria in an animal, causes an increased level of somatic cells in milk. An increased level of somatic cells in milk results in poorer quality milk products which are harder to process. About 80% of the losses attributed to a clinical episode involve the discarding of the nonsalable milk and decreased milk production. Additional losses are incurred by the farmer, such as premature culling and replacement heifer costs, or veterinary services and the cost for drugs. The loss is estimated to be US $184 per episode. In the USA alone, it is noted that over US $ 1 billion is lost in one year due to mastitis. The prevention procedures at milking are less efficient especially when the mastitis is in a subclinical phase and there are no visible signs of the disease. Special efforts have to be made at each milking to detect subclinical mastitis in individual animals before they become clinical episodes.
Milk production is also affected by the presence of environmental mastitis pathogens in animals. Generally, less than 10% of quarters in a herd are infected with environmental mastitis pathogens. Environmental mastitis causes a decrease in milk production but only to a mid level, where the SCC is between 350,000 to 500,000 cells/ml. Statistically, the risk factor for an animal with environmental mastitis pathogens to get infectious mastitis pathogens, is 60%.
Milk composition is influenced by many factors such as soil, feed, and water. It can also vary during milking, during the day, and with the season. The most frequent ions in milk are sodium and potassium ions which are transported passively from the secretory cells into the milk. Chloride ions are also found in milk but they have a higher concentration in the animal's blood and extracellular fluids than in milk. The concentration of potassium ions is relatively low in milk and the concentrations of sodium and chloride ions is relatively high.
Mastitis has a marked effect on milk composition. Generally, ion concentration in mastitic milk is higher than in normal milk. The electrical conductivity is higher in mastitic milk than in normal milk. In normal milk, electrical conductivity is about 3.1 miliSiemens/cm. A high electrical conductivity of milk of about 3.3 mS/cm indicates an infected quarter. The increase of electrical conductivity is due to an increase of sodium and chloride ion concentration.
Mastitis is currently detected by measuring changes in the electrical conductivity of milk. Electrical conductivity is generally measured with a DC or AC circuit having a probe positioned in the flow of milk. The most sensitive part of this on-line method is the probe. The probe generally includes two electrodes to which an AC or DC current is supplied to create an electrical circuit through the milk. The conductivity of the milk is evaluated by measuring the current variations in the circuitry that includes the probe. However, the readings are often inaccurate due to deposits of colloidal materials from the milk on the electrodes, and also due to polarization. Polarization occurs because some of the ions migrating towards the electrodes are not neutralized and consequently, an offset, or leakage current is generated between the electrodes. The presence of the leakage current results in inaccurate conductivity readings.
U.S. Pat. No. 3,762,371 issued to Joshua Creer Quayle et al. in 1973, describes an apparatus and a method for comparing the inductance of liquid streams for detecting mastitis. In this patent the suction teats engaging cup of a milking apparatus has a hemispherical chamber provided with four conductivity measuring cells. Each measuring cell includes a coil. The coils induce currents into the stream of milk from a quarter. The coils are placed in the arms of a four-arm electrical bridge which is balanced before testing. The induced currents change the impedance of the coil, depending on the electrical conductivity of the milk. An imbalance of the bridge during testing is due to variations in milk conductivity.
However, the system described in the above mentioned patent, is somehow complicated and not suitable for on-line measurements. Moreover, the system is based on the prediction that mastitis first occurs in one quarter, and can not detect mastitis occurring simultaneously in two or all quarters.
U.S. Pat. No. 5,416,417 issued to Eli Peles in 1995, discloses a method for determining the onset of mastitis by comparing the electrical conductivity of milk from an individual animal at milking with an average conductivity value previously recorded for the same animal. The average value corresponds to readings made during a predetermined period of time. A deviation between the measured electrical conductivity and the average value is determined at least once a day. Deviations of approximately 15% are considered an indication of the onset of mastitis.
This method does not provide an accurate indication about the type of mastitis or the degree of the infection.
U.S. Pat. No. 5,302,903 issued to Hendrik J. De Jong in 1994, describes a throughflow mastitis detector comprising two electrodes positioned at the bottom of a measuring chamber. The electrodes have a shank with a larger head projecting inside the measuring chamber, above and flush with the bottom surface, to avoid formation of areas where bacteria colonies may develop. This detector is not placed in an optimal sensing area. The milk flow is discontinued and obstructed by the measuring chamber. Moreover, milk fat/protein can build-up around electrodes causing current leakage. Also, cleaning the detector may be difficult.
Accordingly, there is a need for an improved on-line somatic cell analyser.